String in Java is very special class and most frequently used class as
well. There are lot many things to learn about String in Java
than any other class, and having a good knowledge of different String
functionalities makes you to use it properly. Given heavy use of Java String in almost
any kind of project, it become even more important to know subtle detail about
String. Though I have shared lot of String related article already here in
Javarevisited, this is an effort to bring some of String feature together. In
this tutorial we will see some important points about Java String, which is
worth remembering. You can also refer my earlier post 10
advanced Java String questions to know more about String. Though I
tried to cover lot of things, there are definitely few things, which I might
have missed; please let me know if you have any question or doubt on java.lang.String
functionality and I will try to address them here.
1)
Strings are not null terminated in Java.
Unlike C and C++, String in Java doesn't terminate with null character. Instead
String are Object in Java and backed by character array. You can get the
character array used to represent String in Java by calling toCharArray() method of java.lang.String class of
JDK.
2)
Strings are immutable and final in Java
Strings are immutable in Java it means once created you cannot modify
content of String. If you modify it by using toLowerCase(), toUpperCase() or any
other method, It always result in new
String. Since String is final there is no way anyone can extend String or
override any of String functionality. Now if you are puzzled why
String is immutable or final in Java. checkout the link.
3)
Strings are maintained in String Pool
As I Said earlier String is special class in Java and all String literale.g. "abc" (anything
which is inside double quotes are String literal in Java) are maintained in a
separate String pool, special memory location inside Java memory, more
precisely inside PermGen
Space. Any time you create a new String object using String literal, JVM
first checks String pool and if an object with similar content available, than
it returns that and doesn't create a new object. JVM doesn't perform String
pool check if you create object using new operator.
You may face subtle issues if you are not aware of this String behaviour , here is an example
String name = "Scala"; //1st String object
String name_1 = "Scala"; //same object referenced by name variable
String name_2 = new String("Scala") //different String object
//this will return true
if(name==name_1){
System.out.println("both name and name_1 is pointing to same string object");
}
//this will return false
if(name==name_2){
System.out.println("both name and name_2 is pointing to same string object");
}
if you compare name and name_1 using equality operator "==" it will
return true because both are pointing to same object. While name==name_2 will
return false because they are pointing to different string object. It's worth
remembering that equality
"==" operator compares
object memory location and not characters of String. By default Java puts
all string literal into string pool, but you can also put any string into pool
by calling intern() method of java.lang.String class,
like string created using new() operator.
4) Use
Equals methods for comparing String in Java
String class overrides equals method and provides a content equality,
which is based on characters, case and order. So if you want to compare two
String object, to check whether they are same or not, always use equals() method
instead of equality operator. Like in earlier example if we use equals
method to compare objects, they will be equal to each other because they
all contains same contents. Here is example of comparing String using equals
method.
String name = "Java"; //1st String object
String name_1 = "Java"; //same object referenced by name variable
String name_2 = new String("Java") //different String object
if(name.equals(name_1)){
System.out.println("name and name_1 are equal String by equals method");
}
//this will return false
if(name==name_2){
System.out.println("name_1 and name_2 are equal String by equals method");
}
You can also check my earlier post difference
between equals() method and == operator for more detail discussion on
consequences of comparing two string using == operator in Java.
5) Use
indexOf() and lastIndexOf() or matches(String regex) method to search inside
String
String class in Java provides convenient method to see if a character or sub-string or a pattern exists in current String object. You can use indexOf() which will
return position of character or String, if that exist in current String object
or -1 if character doesn't exists in String. lastIndexOf is similar
but it searches from end. String.match(String regex) is even
more powerful, which allows you to search for a regular
expression pattern inside String. here is examples of indexOf, lastIndexOf and matches method
from java.lang.String class.
String str = "Java is best programming language";
if(str.indexOf("Java") != -1){
System.out.println("String
contains Java at index :" + str.indexOf("Java"));
}
if(str.matches("J.*")){
System.out.println("String
Starts with J");
}
str ="Do you like Java ME or Java EE";
if(str.lastIndexOf("Java") != -1){
System.out.println("String contains Java lastly at: " + str.lastIndexOf("Java"));
}
As expected indexOf will return 0 because characters
in String are indexed from zero. lastIndexOf returns
index of second “Java”, which starts at 23 and matches
will return true because J.* pattern is any String starting with character
J followed by any character because of dot(.) and any
number of time due to asterick (*).
Remember matches() is tricky and some time
non-intuitive. If you just put "Java" in matches
it will return false because String is not equals to
"Java" i.e. in case of plain text it behaves like equals method. See here
for more examples of String matches() method.
Apart from indexOf(), lastIndexOf() and matches(String
regex) String also has methods like startsWith() and endsWidth(), which can
be used to check an String if it starting or ending with certain character or
String.
6) Use
SubString to get part of String in Java
Java String provides another useful method called substring(), which can
be used to get parts of String. basically you specify start and end index and substring() method
returns character from that range. Index starts from 0 and goes till String.length()-1. By the
way String.length() returns you number of characters in String,
including white spaces like tab, space. One point which is worth remembering
here is that substring is also backed up by character array, which is used by
original String. This can be dangerous if original string object is very large
and substring is very small, because even a small fraction can hold reference
of complete array and prevents it from being garbage collected even if there is
no other reference for that particular String. Read How
Substring works in Java for more details. Here is an example of using SubString
in Java:
String str = "Java is best programming language";
//this will return part of
String str from index 0 to 12
String subString = str.substring(0,12);
System.out.println("Substring: " + subString);
7)
"+" is overloaded for String concatenation
Java
doesn't support Operator overloading but String is special and + operator
can be used to concatenate two Strings. It can even used to convert int, char, long or double to convert
into String by simply concatenating with empty
string "". internally + is implemented
using StringBuffer prior to Java 5 and StringBuilder from Java
5 onwards. This also brings point of using StringBuffer or StringBuilder for
manipulating String. Since both represent mutable object they can be used to
reduce string garbage created because of temporary String. Read more about StringBuffer
vs StringBuilder here.
8) Use
trim() to remove white spaces from String
String in Java provides trim() method to remove white space
from both end of String. If trim() removes white spaces it
returns a new String otherwise it returns same String. Along with trim() String also provides replace() and replaceAll() method for
replacing characters from String. replaceAll method even
support regular expression. Read more about How to replace String in Java here.
9) Use
split() for splitting String using Regular expression
String in Java is feature rich. it has methods like split(regex) which can
take any String in form of regular expression and split the String based on
that. particularly useful if you dealing with comma separated file (CSV) and
wanted to have individual part in a String array. There are other methods also
available related to splitting String, see this Java
tutorial to split string for more details.
10) Don't
store sensitive data in String
String pose security threat if used for storing sensitive data like
passwords, SSN or any other sensitive information. Since String is immutable in
Java there is no way you can erase contents of String and since they are kept
in String pool (in case of String literal) they stay longer on Java heap ,which
exposes risk of being seen by anyone who has access to Java memory, like
reading from memory dump. Instead char[] should be
used to store password or sensitive information. See Why
char[] is more secure than String for storing passwords in Java for more
details.
11) Character Encoding and String
Apart from all these 10 facts about String in Java, the most critical thing to know is what encoding your String is using. It does not make sense to have a String without knowing what encoding it uses. There is no way to interpret an String if you don't know the encoding it used. You can not assume that "plain" text is ASCII. If you have a String, in memory or stored in file, you must know what encoding it is in, or you cannot display it correctly. By default Java uses platform encoding i.e. character encoding of your server, and believe me this can cause huge trouble if you are handling Unicode data, especially if you are converting byte array to XML String. I have faced instances where our program fail to interpret Strings from European language e.g. German, French etc. because our server was not using Unicode encodings like UTF-8 or UTF-16. Thankfully, Java allows you to specify default character encoding for your application using system property file.encoding. See here to read more about character encoding in Java
That's all about String in Java. As I have said String is very special in
Java, sometime even refer has God class. It has some unique feature like immutability, concatenation
support, caching etc, and to become a serious Java programmer,
detailed knowledge of String is quite important. Last but not the least don't
forget about character
encoding while converting a byte array into String in Java. Good knowledge of java.lang.String is must for good Java developers.
Source:http://javarevisited.blogspot.com/2013/07/java-string-tutorial-and-examples-beginners-programming.html
Tidak ada komentar:
Posting Komentar